1. Field of the Invention
The present invention relates generally to waveform measurements and, more specifically, the present invention relates to the measurement of electrical waveforms proximate to the surface of a sample such as for example an integrated circuit.
2. Background Information
The microelectronics field is a multi-billion dollar industry that is driving rapid technological advances in the fabrication of dense high frequency integrated circuits. Within this industry there is a continuing effort to increase integrated circuit speed as well as device density. The continuing technological advances create major challenges for researches in the test and measurement field. For instance, the ability to measure the internal signals of a circuit is often important in order to perform design test, diagnostics and failure analysis of advanced microelectronics. High spatial and temporal resolution, non-invasiveness, and accuracy are among the desirable characteristics of a suitable measurement instrument. As circuit operating frequencies continue to increase and as device dimensions continue to decrease, these desirable characteristics of measurement instruments become increasingly difficult to achieve.
Present day measurement and probing methods based on direct physical contact of internal signal lines of a chip are often not suitable for internal testing of many circuits due to probe contact area, spatial limitations and/or the parasitic loading caused by the probe. Other disadvantages associated with probing methods based on direct physical contact of internal signal lines of a chip include circumstances where test points are not readily accessible, the electrical contacts of the test points are unreliable, the direct contact probe tip sizes are excessive in size and the removal of passivation layers are necessary to expose the test points.
Several non-contact measurement techniques, however, have been developed as alternatives to direct contact probing. Among the non-contact measurement techniques are electro-optic probing, opto-electronic sampling, reactive near-field probing, high-speed photoemission sampling, and electron-beam testing. Several of these methods are capable of providing very high spatial and/or temporal resolution. However, these methods rely on measuring a secondary effect of the local circuit potential, that require complex calibration procedures thereby making accurate voltage measurements difficult and in some cases impossible. Furthermore, many of the above-listed instruments also require very specialized operating environment. For example, electron-beam testing must be performed in a vacuum.
In the last decade, several new measurement techniques based on scanning force have been utilized for non-contact measurements. One of the instruments that use the new measurement techniques is the electrostatic force microscope (EFM), which is capable of measuring static or low frequency voltages on integrated circuits with very high spatial resolution. An advantage of the EFM approach is that it is simple and can be performed in air over passivated circuits. Until very recently, however, the technique has been limited to measuring only signals at repetition rates at, near or below the mechanical response of the probe, which is typically less than 100 KHz. As can be appreciated, many microelectronic measurement applications require higher frequency measurement capabilities in order to be useful.
A number of new techniques have been proposed to overcome the frequency limitations of the EFM. These new techniques have been used with varying degrees of success in proposed non-contact probing instruments that are able to perform both high speed digital signal and high frequency vector analog signal measurements. However, a number of difficulties and disadvantages still remain with the proposed probing instruments. For instance, accurate voltage measurements require precise probe positioning and calibration at every measurement location. The proposed probing instruments are subject to non-repetitive direct current (DC) voltage offset effects. Long bit pattern measurements are subject to signal-to-noise reduction. The equivalent-time bandwidth is limited at high frequencies due to probe mechanical frequency response and is subject to noise at low frequencies. Furthermore, the proposed techniques require expensive equipment and/or electronic components to implement or are band limited due to the unavailability of wide band components. Moreover, the need for calibration makes it difficult to use some of the above-listed methods for passivated circuits.
Therefore, what is desired is a method and an apparatus for providing measurement of electrical waveforms from a sample such as for example an integrated circuit. Such a method and apparatus should measure the electric signals from the surface of the sample without coming in direct physical contact with the sample. In addition, such a method and apparatus should provide measurement of high frequency signals without the need for expensive equipment and/or electronic components to implement.